This invention relates to xerography and more specifically to an improved photosensitive material for use in binder layer xerographic photoreceptors.
The art of xerography involves the use of a photoconductive element or plate which is uniformly electrostatically charged in order to sensitize its surface. The plate is then exposed in an imagewise manner to activating electromagnetic radiation which selectively dissipates the charge in the exposed areas of the photoconductive material while leaving behind a latent electrostatic image in the non-exposed areas. This latent electrostatic image may then be developed by depositing a finely divided, electroscopic marking material on the surface of the photoconductive material. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described in many related patents.
One type of photoconductive layer used in xerography is described in U.S. Pat. No. 3,121,006 to Middleton and Reynolds which describes a number of binder layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an organic, electrically insulating resin binder.
In the particular examples of the binder systems described in Middleton et al, the dispersion of photoconductive particles is relatively uniform, such uniformity having been accomplished by thorough mixing of the resin and photoconductive particles. With these uniform dispersions a relatively high volume concentration of photoconductor particles, usually about 50 percent by volume, is used to obtain the requisite particle-to-particle contact necessary for rapid discharge. This type of binder layer is quite useful for non-reusable systems where the photoreceptor is applied to the paper, but does not have the necessary physical properties to be useful in high speed cyclic imaging.
It has been discovered more recently that the optimum volume concentration of photoconductive material can be reduced significantly, i.e. to a level of from 1 to 25 volume percent, without sacrificing photosensitivity by controlling the bulk geometry of the photoconductive binder layer to insure particle-to-particle contact of the photoconductive particles throughout the thickness of the binder layer. This reduction in concentration of the photoconductive particles results in enhanced mechanical and surface properties as well as improved control of the electrical characteristics of the binder layer. This concept, which is more fully described by R. N. Jones in U.S. Pat. No. 3,787,208, uses binder materials in particulate form which have been classified to achieve a restricted particle size and particle size distribution. A mixture of these particles in the proper proportion may then be dispersed in a suitable carrier media in which neither constituent is soluble. A continuous film may then be formed from this suspension, dried, and the binder particles fused together. The critical step is not the application procedure but the photoreceptor geometry control achieved by employing a particulate binder and a particulate photoconductive material having the correct size and size distribution. The photoconductive material is preferably of sub-micron size to achieve the desired configuration. By effective application of this controlled size particulate binder concept, it is possible to utilize a wide choice of binder materials in order to achieve the desired physical properties.
There is an ongoing need for improved photoconductive materials for use in the above-described binder layer photo-receptors. In the synthesis of group II chalcogenides for employment in binder layer photoreceptors it is possible to employ well-known doping techniques to achieve photosensitivity much greater than that of zinc oxide. These preparation methods require the incorporation into the lattice of small quantities of elements such as copper, chlorine, gallium, aluminum, etc., which is normally accomplished by multiple firing of the materials in the presence of a fluxing mineralizing agent. Resistive, highly sensitive photoconductors have been prepared in this manner, however, this method inherently leads to a relatively large particle size, 1-30 .mu., which places severe limitations on the use of these materials in fabricating electrostatographic photoreceptors. This is especially true in terms of the resultant high background density and limited mechanical properties encountered with layers having a high volume loading of photo-conductor, and poor sensitivity in geometry controlled matrix layers where the volume concentration of the photoconductor phase is reduced.
It is well known that cadmium chalcogenides, e.g. CdS, CdTe, CdAs, CdSe and CdSSe, manufactured as pigments are photosensitive and can be utilized in the fabrication of xerographic layers. These materials possess an advantage in that they can be prepared in a particle size which ranges from 0.001 to 1 .mu. and are, therefore, eminently more suited to the formulation of xerographic photoreceptors than presently available doped materials. Two major problems have restricted the use of these materials in the past. They are: (1) a comparatively low order of sensitivity as compared to the doped materials and (2) the inability to consistently produce a material having the same electrical characteristics by presently known processes.
Sodium sulfide solutions have been used extensively for many years in the production of cadmium sulfide by reacting the sulfide solution with a soluble cadmium salt to bring about the precipitation of the cadmium in the form of its sulfide. Similarly, other cadmium chalcogenides such as the telluride, arsenide and selenide can be prepared. Calcination of the precipitate results in the formation of a photoconductive material. In the process of the instant invention, finely divided CdO or a material which is thermally degradable to CdO, i.e. oxidic cadmium, is added to the precipitate before calcination.
Cadmium sulfoselenide, sometimes referred to as cadmium red pigment, can be prepared by mixing cadmium sulfide, cadmium oxide and selenium in finely divided form and calcining the mixture at an elevated temperature to form the desired product. Such a process is disclosed in U.S. Pat. No. 2,134,055 wherein it is stated that the presence of cadmium oxide during calcination is advantageous because it reduces or eliminates losses of selenium. The patentee goes on to state that "it is often desirable to use somewhat less cadmium oxide than the specified molecular proportions in order to avoid the possibility of any unreacted cadmium oxide in the final product."
The foregoing patent stresses the desirability of using less than a stoichiometric amount of selenium and CdO or an oxidic cadmium compound. This is understandable since the patentee was interested in the preparation of CdSSe for use as a pigment and the presence of unreacted CdO in the product would be undesirable.
Preparation of CdSSe by the calcination of a material containing cadmium sulfide, selenium and cadmium oxide or an oxidic cadmium compound will in some instances provide a product having a particle size in the sub-micron range desired for use in binder layer photoreceptors. However, it has been discovered that CdSSe prepared by prior art methods is not particularly suitable for use in binder layer photoreceptors especially those of the controlled geometry type in which the volume concentration of photoconductor is relatively low for the reasons previously set out.
It would be desirable and it is an object of the present invention to provide a method for improving the photo-induced discharge characteristics of certain cadmium chalcogenides.
An additional object is to provide a novel process for the preparation of cadmium sulfoselenide which exhibits improved photo-induced discharge characteristics.
It is a further object to provide such a process in which the cadmium sulfoselenide is prepared by the calcination of cadmium sulfide, cadmium oxide and selenium in finely divided form.
An additional object is to provide such a process which produces a CdSSe pigment suitable for use in binder layer electrostatographic photoreceptors.
Another object is to provide such a process which produces a CdSSe pigment suitable for use in geometry controlled binder layer photoreceptors.